This, it turns out, is the technique that was used to blame Navy Gunner's Mate Clayton Hartwig for the explosion that killed him and 46 shipmates aboard the battleship USS Iowa last April.

In an afternoon of eye-popping testimony before the Senate Armed Services Committee last week, Richard Ault of the FBI's National Center for the Analysis of Violent Crime explained the operation: The Navy asked for the FBI's assistance in solving the Iowa mystery - but told the FBI that it had already ruled out accident.

Further, if the explosion was not an accident, the Navy said, it must have been an act of sabotage, suicide or murder. The FBI also accepted that. Also, the Navy said, if it was sabotage, suicide or murder, Hartwig was the only person who could have done it. With these marching orders, the FBI went to work and found Hartwig guilty.

Sen. Alan Dixon, D-Ill., was incredulous. He asked the FBI's Robert Hazelwood: ''You interviewed nobody, analyzed no fingerprints, analyzed no handwriting, otherwise did any investigation concerning an accident, an explosion, friction which can cause an explosion . . . none of that was done?''

''We don't do that type of investigation in this type of request, sir,'' Hazelwood replied.

Nor did the FBI investigate the possibility that someone else might have set off the explosion.

''As professionals, wouldn't it have been of interest to you to make an assessment of whether there were other individuals?'' Cohen asked.

''No, sir,'' said Hazelwood. ''Whenever we are requested to do a case for an investigative agency, we make the assumption we are dealing with professional investigators.'' The Navy, even as it asked the question, said it wanted only one answer - Hartwig - and the FBI went to work.

''That's a pretty narrow band of examination,'' an astonished Cohen said. ''The Navy came to the FBI with a pre-ordained conclusion, and the FBI comes back with the Good Housekeeping Seal of Approval.''

Oh, there was a pretense of analysis. The FBI read the evidence the Navy provided - chiefly interviews conducted by the Naval Investigative Service and letters from Hartwig.

On the basis of the Navy's ''evidence'' - that Hartwig was a loner, was allegedly fascinated by violence and death, once collected knives, had only three women friends but owned seven Bibles - the FBI psychologists reported back to the Navy that Hartwig ''did, in fact, commit suicide and hoped that this death would appear to be an accident.''

The Navy then cited the FBI's opinion as one of the ''facts'' indicating that Hartwig was probably guilty of blowing up the Iowa.

When grilled by the senators, the FBI said its conclusion was not a ''fact'' but an opinion.

Dixon asked Ault: ''Do you have any hard evidence, any evidence that would support the idea that Hartwig actually carried out this act?''

Ault replied: ''No, sir. This opinion that we submitted is based on a half-scientific, half-art form. . . . We're not detectives ourselves. . . . We're professionals in the area of behavioral sciences.''

Even though the FBI witnesses said they are accepted as experts in criminal proceedings, any defense lawyer, any judge, any jury would laugh this kind of quack evidence out of court. One hopes.